The Universal Joint Assembly for a down hole drilling motor must accommodate substantial rotary input torque and thrust from the oscillating and rotating member of the power section and translate this motion and torque to an axially uniform rotation for the bearing assembly. A typical universal joint assembly may have zero, one or two movable ends to accommodate this motion from the power section. In the case of movable ends, common methods include thrust ball(s) or spherical end(s) on the ends of the universal joint shaft for the axial thrust and drive balls arranged about the outside circumference on one or both ends of the universal joint shaft for rotational loads. The drive balls are allowed to move axially in the cylindrical recesses of their mating parts. This allows the shaft to pivot angularly while delivering rotational torque through the universal joint system.
However, a capacity limitation arises in the drive ball and cylinder method utilized in this method. The drive ball has a spherical shape and when mated with a cylinder, the contact between the two components is a “line” of contact between mating surfaces. In theory, neglecting material deformation, the surface area of contact between these mating surfaces is zero. When high torque loads are applied to these surfaces, the contact stresses quickly exceed their material limits and the surfaces begin to wear. As wear progresses, the movable end(s) of the universal joint begin to “bind” which results in the development of bending stresses in the system. These bending stresses often lead to failures.
A universal joint known in the prior art is shown in FIG. 1-6 and is generally indicated by reference numeral 100. Referring to FIG. 1, universal joint 100 has an inner race 102 and an outer race 104. Inner race 102 is locked within outer race 104, but is permitted to pivot omnidirectionally. Inner race 102 has a series of semi-spherical cavities 106 while outer race 104 has a series of corresponding channels 108. As shown in FIG. 4, channels 108 have a semi-circular cross-section. Channels 108 are oriented along the direction of the axis of the shaft 112 on which universal joint 100 is carried. Referring to FIGS. 5 and 6, torque transfer elements 110 are spherical in shape such that a portion engages cavities 106 and another portion engages channel 108. Torque transfer elements are able to move along channels 108 while permitting relative pivotal movement of inner race 102 and outer race 106. The shape of inner and outer races 102 and 104 may vary. As shown in FIG. 2, inner race 102 may be square and pivot around a central pivot point 114, or, as shown in FIG. 3, may be rounded and pivot as a ball joint. The universal joint design described below may be used with these or other designs.